Diesel pollution control devices include catalytic converters and diesel particulate filters or traps. The pollution control devices typically comprise a metal housing or casing with a pollution control element securely mounted within the casing by a resilient and flexible mounting mat. Pollution control devices are universally employed on motor vehicles to control atmospheric pollution. Two types of devices are currently in widespread use: catalytic converters and diesel particulate filters or traps. Catalytic converters contain a catalyst, which is typically coated on a monolithic structure mounted within a metallic housing. The monolithic structures are typically ceramic, although metal monoliths have also been used. The catalyst oxidizes carbon monoxide and hydrocarbons and reduces the oxides of nitrogen in automobile exhaust gases to control atmospheric pollution.
Diesel particulate filters or traps are typically wall flow filters, which have honeycombed, monolithic structures typically made from porous crystalline ceramic materials. Alternate cells of the honeycombed structure are typically plugged such that exhaust gas enters in one cell and is forced through the porous wall to an adjacent cell where it can exit the structure. In this way, the small soot particles that are present in diesel exhaust gas are collected.
The monoliths and in particular the ceramic pollution control monoliths, used in pollution control devices are fragile and susceptible to vibration or shock damage and breakage. They have a coefficient of thermal expansion generally an order of magnitude less than the metal housing which contains them. This means that as the pollution control device is heated the gap between the inside peripheral wall of the housing and the outer wall of the monolith increases. Even though the metallic housing undergoes a smaller temperature change due to the insulating effect of the mat, the higher coefficient of thermal expansion of the metallic housing causes the housing to expand to a larger peripheral size faster than the expansion of the monolithic element. Such thermal cycling occurs hundreds of times during the life and use of the pollution control device.
To avoid damage to the ceramic monoliths from for example road shock and vibrations, to compensate for the thermal expansion difference, and to prevent exhaust gases from passing between the monolith and metal housing (thereby bypassing the catalyst), mounting mats are disposed between the ceramic monolith and the metal housing. These mats must exert sufficient pressure to hold the monolith in place over the desired temperature range but not so much pressure as to damage the ceramic monolith.
Many of the mounting mats described in the art have been developed for mounting catalytic converters for treatment of exhaust from gasoline engines which typically operate at high temperature. Known mounting mats include intumescent sheet materials comprised of ceramic fibers, intumescent materials and organic and/or inorganic binders. Intumescent sheet materials useful for mounting a catalytic converter in a housing are described in, for example, U.S. Pat. No. 3,916,057 (Hatch et al.), U.S. Pat. No. 4,305,992 (Langer et al.) U.S. Pat. No. 5,151,253 (Merry et al.) U.S. Pat. No. 5,250,269 (Langer) and U.S. Pat. No. 5,736,109 (Howorth et al.). In recent years, non-intumescent mats comprised of polycrystalline ceramic fibers and binder have been used especially for the so-called ultra thin-wall monoliths, which have significantly lower strength due to their extremely thin cell walls. Examples of non-intumescent mats are described in, for example, U.S. Pat. No. 4,011,651 (Bradbury et al.), U.S. Pat. No. 4,929,429 (Merry), U.S. Pat. No. 5,028,397 (Merry), U.S. Pat. No. 5,996,228 (Shoji et al.), and U.S. Pat. No. 5,580,532 (Robinson et al.). Polycrystalline fibers are much more expensive than normal, melt formed ceramic fibers and, therefore, mats using these fibers are only used where absolutely necessary as, for example, with ultra thin-wall monoliths.
U.S. Pat. No. 5,290,522 describes a catalytic converter having a non-woven, mounting mat comprising at least 60% by weight shot-free high strength magnesium aluminosilicate glass fibers having a diameter greater than 5 micrometers. The mounting mats taught in this reference are primarily intended for use in high temperature applications as can be seen from the test data in the examples where the mats are subjected to exhaust gas temperatures of more than 700° C.
U.S. Pat. No. 5,380,580 describes a flexible non-woven mat comprising shot-free ceramic oxide fibers selected from the group consisting of (a) aluminosilicate fibers comprising aluminum oxide in the range from 60 to about 85% by weight and silicon oxide in the range of 40 to about 15% by weight silicon oxide, based on the total weight of said aluminosilicate-based fibers, said aluminosilicate-based fibers being at least 20% by weight crystalline (b) crystalline quartz fibers and (c) mixtures of (a) and (b), and wherein the combined weight of said aluminosilicate-based fibers and said crystalline quartz fibers is at least 50% by weight of the total weight of said non-woven mat. The flexible non-woven mat can additionally comprise high strength fibers selected from the group consisting of silicon carbide fibers, silicon nitride fibers, carbon fibers, silicon nitride fibers, glass fibers, stainless steel fibers, brass fibers, fugitive fibers, and mixtures thereof.
Diesel Oxidation Catalysts (DOC's) are used on modern diesel engines to oxidize the soluble organic fraction (SOF) of the diesel particulate emitted. Because of extremely low exhaust gas temperature, mounting of DOC's with conventional mounting materials has been problematic. The exhaust gas of modern diesel engines such as turbo-charged direct injection (TDI) engines may never exceed 300° C. This temperature is below the temperature needed to expand most intumescent mats. The expansion is needed to develop and maintain appropriate pressure within the catalytic converter. Additionally, this temperature is too low to burn out the organic binder contained in intumescent mat materials. At these temperatures the binder only softens, which acts to interfere with the resiliency of the ceramic fibers. As a result, field failures have occurred with DOC's when using conventional intumescent mounting mats. To overcome these difficulties, the converters are sometimes heat treated prior to installation to expand the vermiculite and burn out the binder. This is expensive and time consuming. Auxiliary wire mesh “L” seals have also been employed to augment the holding force of intumescent mats at low temperature, but also add cost and complexity to assembly. Most non-intumescent mats while performing somewhat better still contain an organic binder, which significantly reduces the resiliency of the fibers in the 200-300° C. temperature range. This is expected to be true for other diesel exhaust pollution control devices, as well, including lean NOx catalysts, Continuous regenerating traps (CRT's) and NOx traps.
U.S. Pat. No. 6,231,818 attempts to overcome the present difficulties of mounting low-temperature, diesel catalysts by using non-intumescent mats comprised of amorphous, inorganic fibers. Although it is taught in this patent that the mat can be organic binder free, it appears that several of the mats used in the examples require the use of substantial amounts of binders. Moreover, it was found that the mounting mats disclosed in this U.S. patent, still do not adequately perform for treatment of exhaust from diesel engines, in particular TDI engines.
It was thus desirable to find an alternative mounting mat for mounting a diesel pollution control monolith in the metallic casing of a pollution control device for the treatment of exhaust from a diesel engine. In particular, it was a desire to obtain such improved mounting mats that can be manufactured in an easy and convenient way at an affordable cost. Additionally, it was a desire to find mounting mats that show good to excellent performance in at least one or more of the following tests Real Condition Fixture Test (RCFT), Cyclical Compression Test, and Hot Vibration Test. Desirably, the mounting mat is also more acceptable in terms of health, safety and environmental aspects.